Lithography is a process used in semiconductor manufacturing. Desired ICs are designed as “design patterns” that include geometric features (e.g., lines, dots, and shapes) representing the circuits, and the design patterns can be transferred onto a substrate (e.g., a silicon wafer) in an optical process followed by physical and chemical processes (e.g., exposure, development, or etching) to turn the transferred patterns (referred to as “substrate patterns”) into a manufactured circuit. The optical process and the following physical and chemical processes can be integrally referred to as a “pattern transfer process.”
In the pattern transfer process, the design patterns can be transferred to a “photomask” (or “mask”) before being transferred to the substrate. The process of transferring the design patterns onto the mask can be termed as “mask making” or “mask writing.” The transferred patterns on the mask (referred to as “mask patterns”) can be transferred to the substrate by illuminating the same using a light source (e.g., an ultraviolet or UV light, a deep ultraviolet or DUV light, or an extreme ultraviolet or EUV light) and projecting the transmitted light onto the substrate for exposure using an optical system.
Due to the optical resolution limitations in the pattern transfer process, the substrate patterns can have distortions compared with the mask patterns. Due to uncertainties introduced in the physical and chemical processes, defects in the substrate patterns can also occur. Based on a relationship between the mask patterns and the substrate patterns, various resolution enhancement techniques (RETs), such as optical proximity correction (OPC), can be used for the mask writing to pre-compensate and minimize such distortions and defects. However, as the semiconductor manufacturing approaches smaller nodes (e.g., 14 nanometers or below), the OPC faces more challenges due to the increasing complexity of the mask patterns.